Multihull Structure Thoughts

“The Gemini “Freestyle” 37 is a completely new concept in boat design and customization” is the line used in the advertising. But the design looks familiar. Think about the Gemini 31, Gemini 105 Mc and the Gemini Legacy especially the version that was extended to 37 foot (first jpeg). Now the extended Gemini Legacy mold is being used to create the Gemini “Freestyle” 37. Alternative deck molds to suit various purposes such as a day charter catamaran, a cruiser or a “luxury” catamaran can be added. The Gemini “Freestyle” 37 day charter is 37.1 x 14 foot weighing 7,500 lbs and displacing 9,800 lbs. The 40.5 foot mast carries about a 385 square foot mainsail and a 270 square foot genoa. The hull length to beam is about 10 to 1. This would be a safe reasonable cruising design, not a high performance design.

Over the near 30 years of production, the Gemini catamarans have evolved structurally. The Gemini Freestyle 37 has Blister prevention hull technology with a fiberglass rib grid reinforcing a solid fiberglass bottom from the waterline down, including the low aspect ratio keels. Above the waterline the structure is either a fiberglass Balsa sandwich or (probably for flat panels) Naidacore© glass sandwich depending on the structure.

The day charter version mainly featured here has a forward cabin with a hanging locker, queen size bed and storage shelving. There is also a separate toilet forward.

The basic Gemini “Freestyle” 37 can be a charter machine or for additional money turned into a cruiser or a “luxury” cat. The charter machine is the cheapest “initial cat” that can be evolved to what ever you want at a price.

An alternative version is listed in the last jpeg which is a 399 power catamaran based on the same hull as above but with a further extended hull for the big outboards but the same beam etc. The jpegs give the idea.

 

This one is about a big charter cat. This cat is designed to carry people to an island for entertainment, not about high performance. The cat is called Aneecha (translation from Balinese “Winds of Change”) and is 80 length overall including it bow prodder and 73 foot on deck x 45 foot beam. The displacement is 41,000 lbs (I think this is the weight but cannot confirm) and it carries a schooner rig with a 56 foot main mast and a 50 foot fore mast. The total sail area in the 3 headsails and 2 mainsails is about 1460 square foot. The hull length beam is about 12 to 1. Aneecha’s claimed performance is between 6 to 10 knots for charter work.

The concept designer and builder is Martin Moore with the support of a naval architect, Paul de Saint Front, who developed line plans, along with a weight schedule, structure layout (without structure calculations) and a construction plan. The cat was built at Mr Moore’s Balinese boat yard. The construction is mainly a fiberglass balsa sandwich.

So why the interest? When you look at the jpegs. This is a case of a set of high speed hulls combined with a slow upwind rig. The underwing clearance and shape is appropriate for calmer waters but I suspect would pound in in waves upwind. The large central retractable rudder hung of the central underwing deck pod is interesting but not unusual as Kelsall has used this approach on large cats successfully before. The minimal draft, low aspect ratio keels indicate this cat depends on its two 200 HP engines if the winds are against it. The lack of windward ability is not a problem for a charter cat designed to meet a schedule.

Even from very early sketches the rig is as much an artistic creation, as it is a practical solution. It is easy rig to handle as you can drop a sail instead of reefing a sail. This approach reduces the need for big winches but requires more deck equipment and rigging to replace it. I often think a multiple mast rig (of the same materials EG aluminum mast) ends up costing the same, if not more, than a sloop rig of the same sail area with powerful winches and strong deck gear.

This cat is mainly a day charter machine but has 4 double cabins if you want to charter it for longer periods.

Overall, this is a demonstration of a boat to suit a purpose that has some interesting aspects. If it had a higher underwing clearance, a simpler rig concept and deeper keels or daggerboards this could make a nice cruiser if you had the money. But as we find out tomorrow the designer has some more interesting designs for this thread.

The jpegs give an idea.

 

Octopus is a coastal explorer catamaran that can be dismounted for transportation and is trailerable. The designer is Paul de Saint Front from Creative Designs which is in Bali. Paul originally moved from France in 2006. The cat is 26.3 x 18 foot that can be disassembled into an 8 foot wide package for trailing. This is not a “trailer sailor” but more a transportable cat. The weight of the Octopus is 2000lbs and judging by the hulls probably a displacement of 3500 lbs. The rig is a fractional with a 34 foot mast (can be wood, aluminum, carbon etc.) carries a 270 square foot mainsail and a 105 square foot jib. The hull length to beam is 8.6 to 1. The low aspect ratio keels (which are bolted onto the hull) draw 2 foot. The rudder is only 1 spade rudder on one hull.

The Octopus will be a good performance cruiser not a high performance racer cruiser based on the above numbers. To improve this cats speed you could install a dagger board in one hull and remove the rather small low aspect ratio keels. The tacking angles would be reduced 10 or 15 degrees which would improve the VMG (velocity made good) by quite a bit. The next improvement would be a really good set of sails and a gennaker. With these upgrades the design should be a really good performance cruiser.

The hulls are just wide enough to accommodate a “honeymoon” double berth of 3.9 foot with a single in each hull. One hull has a galley the other a large toilet area. The headroom is 5.75 foot. The bridge deck has a large central seating area which can act as the dinette in reasonable conditions.

As this design can be home built, the hull structure of the cat can be of wood strip planking, sandwich balsa/fiberglass, sandwich foam/fiberglass, solid fiberglass/polyester, solid fiberglass/epoxy or cold molded wood. There will be weight differences between each option but it will not be to large. As Richard Woods says, solid fiberglass hulls in smaller size multihulls is not too much of a weight penalty as the skins on either face of a EG foam sandwich need to be thicker for puncture resistance. Result the 2 skins are only slightly less than a single skin. PS the building time will not be to different either as a solid single skin will require some reinforcements that a foam glass structure will not. The deck structures can be plywood/glass, plywood/wood veneer, sandwich balsa/fiberglass or sandwich foam/fiberglass. In short, build in any combination you want eg solid glass below the waterline and EG foam glass above with a wood epoxy deck. Build in the materials that you have experience in and/or want to use.

This appears to be a good performance coastal cruiser. The jpegs give the idea.

 

Attunga 20 is a 20 x 10 foot day sailing catamaran which is subject to some discussion on the “Where plans go to die” thread. The cat weighs about 500 to 600 lbs and carries a 235 square foot low aspect ratio sloop rig. The hulls length to beam ratio is 10 to 1. The hulls were meant to “plane” when sailing fast. The cat performed quite well in stronger winds and when well built could handle some pretty rough weather when day sailing. The guys wanted to know some detail of the structure so here its is.

The cat built of ply and timber framing and stringers. The 600 mm wide planning hulls built from 3 mm ply with 4.5 mm ply bulkheads. The keel is 40 x 30 mm with chines 30 x 15 mm. The gunnels and stringer on the bottom V panel are 19 x 19 mm. The dagger cases had 6 mm ply sides with 25 x 40 mm vertical logs at either end. The decks are 4.5 mm ply with several 19 x 19 deck stringers. The bridge deck is 4.5 mm ply with 50 x 15 mm under wing stringers. The daggerboards were WRC fore and aft with oregan strips in the centre. The early hulls had no fiberglass but were well painted on good ply and timber.

Now the point of interest. The cross-beam structures which allow the cat to be folded to allow trailing. The main beam halves are permentaly attached to the hulls. The forward main beam halves are an aft I beam with an attached D front section. The I beam has a 4.5 mm ply web with a 52 x 25 timber both sides top and bottom. Attached to the front of the I beam is a 4.5 mm ply top and bottom wing section with 12 mm ply wing frames on the folding end. The 2 hull beam half sections are joined by metal brackets and bolts. The metal brackets are 500 mm long x 22 mm wide x 3 mm thick SS strap with a 15 mm bolt hole drilled through it at one end. There is a strop on the top and bottom of the I beam and on both sides of the I beam (a total of 4 straps on 1 half). The straps are screwed into the I beam timbers at 30 mm centres. To fold the cat take out the top bolt and fold on bottom bolts. The rear beam again is a I beam with a 6 mm ply web with 30 x 15 mm timber top and bottom on both sides and has 15 x 45 mm bulkheads on each face as there is no forward D section. The connecting straps are 350 x 30 x 1.2 mm with a 10 mm bolt hole at one end. The straps are again attached by screws into the top and bottom I beam timbers.

There are a few attached jpegs of the Attunga. The final jpeg is of an early prototype of the Tornado with a hard folding deck which was the way the Attunga folded. I may be able to get some jpegs of actual Attunga beam build later after I speak to another person.

 

Aerion is a forward cockpit cruising catamaran by Bali based Paul de Saint Front from Creative Designs. The cat is 45.9 x 24.9 foot that weighs 21,300 lbs and probably displaces 28,000 lbs. The 62.5 foot mast can be wood, aluminum, carbon fibre tubes or a wing mast. The mainsail is 795 square foot, a 390 square foot self tacking furling jib and a 770 square foot genoa. The length to beam of the hulls is 8.65 to 1. The 6 foot wide at the gunnel (5.25 foot at waterline) hulls have carbon fiber foam daggerboards that draw 7 foot. There are carbon fiber foam spade rudders on the hulls. The minimum underwing clearance is 3 foot.

If the sail area and displacements are achieved the performance of this cat will quite good and it will be able to outperform most French cats of the same size. I suspect Chris Whites Atlantic 46 with its narrower hulls would outsail the Aerion design but it would be to find out in the real world.

The cat has a forward cockpit which is accessible from the main cabin. The forward cockpit can accommodate seven people and can be sealed in rough weather. There is no “inside” steering station in the main bridgedeck cabin. The Aerion has full headroom throughout and its galley and dinette are in main bridgedeck cabin. There are 6 double beds, or 4 double beds and 4 single beds depending on your needs. I suspect the bow doubles are marina berths only. Ruth Wharram is the only person I know who could sleep in a forward bow berth upwind in rough weather.

As this design can be home built, the hull structure of the cat can be of wood strip planking, sandwich balsa/fiberglass, sandwich foam/fiberglass or cold molded wood construction. The bulkheads can be wood or foam glass with carbon fibre reinforcements. The decks, roof and deck elements are sandwich balsa/fiberglass or sandwich foam/fiberglass. The resins can be vinylester or epoxy.

This is a good concept which could be a sensible global performance cruiser. The jpegs give the idea.

 

Hi Oldmulti, firstly, like everyone else on here, appreciate your efforts in supplying priceless information to so many, Cheers.
Do you have the Buccaneer 33 plans on media fire? i have found the 28 but would greatly appreciate the 33. thanks again

 

Bucfan, there is some info about the Buccaneer 33 here -
https://sailboatdata.com/sailboat/buccaneer-33-crowther

 

Bucfan. Please look at the PDF's on Page 7 (bucc 33 kick up rudder plans), Page 8 (Bcc 33 Foam glass hull lines and ply hull plans) and Page 10 (Bucc 33 deck and cross beam plans) and probably some where else in this thread (Multihull Structure Thoughts). There is a complete set of Buccaneer 33 plans across several pages but no written instruction booklet which was lost by the plan provider. So its not just in one spot as it would be on EG Media Fire. There is also a complete set of Farrier Command 10 plans and Buccaneer 24 plans as well. Please look at the first page "index" on the first post of this thread for their locations.

 

thanks, i will keep looking, i found some of the 33 plans but not all thanks again

 

Hi I only see the spindrift 37 rudder details on page 7 .. is the buc 33 the same? Thanks.

 

Scuff. Page 7 item 101 PDF entry Bucc 33 kick up rudder. For both Bucfan and Scuff, Crowther early plans were not many sheets and a detailed build book. He just did EG 6 or 8 plan sheets which had multiple information on each sheet. There was a 4 or 5 page build instruction "log" with a materials list normally (not found for Bucc 33). A builder was expected to look thru the plans and interpret or scale off some specs etc. The down side of this threads PDF's are they are not full size as were some of the original plan sheets. If taken to a print expert with some basic dimensions the printer will be able to print off a full size hull line plan etc. Or you can scale the information off a PDF and get close enough. The final tough is if you were seriously intending to build a Bucc 33, I would review some of the newer designs which have more up to date construction especially in the cross arm structures and would be easier to build. EG Team scarab 32 foot tri plans which can be purchased for $150 australian (about $110 US) that are full plans and done in foam glass. The Bucc 33 is a really good tri but it was designed over 40 years ago. The Spindrift 37 rudders are similar but heavier in construction.

 

A small discussion about some current and future boat building products. The following 2 fabrics can be brought today as a substitute for EG E glass, S glass or carbon fibre.

The first is Basalt fabric. So what is Basalt? This is literally an extraction of basalt rock with only a few additions. It has a better temperature, chemical and UV resistance than E or S glass and is compatible with most common polyester, vinylester and epoxy resins. The properties of Basalt are: A: tensile strength about 30% stronger than e-glass depending on manufacturer, about as strong as S glass. B: the same as to 25% weaker in tension than carbon fibre depending on manufacture. C: slightly heavier (about 3%) for a given tensile strength of e or s-glass. D: For a given tensile strength carbon fibre is about 55% of the weight. E: Basalt has better modulus than e or s glass but is lower than carbon fiber. F: Basalt has lower elongation characteristics the E or S glass but stretches twice as much as carbon.

Translation/generalisation. Basalt is nearly as strong as S glass but slightly (about 3%) heavier, it is slightly stiffer and has less elongation than e or s glass. Carbon fiber has in most cases significantly better strength to weight ratios. The cost is higher than e glass but less than carbon fibre. A good material that can be worked the same way as fiberglass eg hand laid or vacuum bagging etc.

The second is a flax based fibre material Bcomp for composite layups. A Swiss company Bcomp has developed the flax base fibre material for use in an epoxy or polyester resin matrix. The final product is like a unidirectional or biaxial cloth that can be used with polyester or epoxy resin. The strength can be higher than polyester e glass but it will be light for a given stiffness. The product was developed because its manufacture only produces about 3% of the carbon emission of the production of carbon fibre fabrics.

Bidirectional fabric with fibers oriented at 0° and 90°, suitable for manufacturing fiber reinforced composite products with a high performance and a low environmental impact. ampliTex® 5040 has a very good drapability and is ideal for complex shapes. High laminate stiffness is obtained due to the low crimp twill 2/2 weave.

Performance advantage. Considering that glass fibers have a density of 2.6 kg/dm3 and a tensile modulus of 70 GPa, the flax ampliTex® 0°/90° 300 gsm can replace a 495 gsm glass fiber 0°/90° fabric to have the same stiffness in tension. In compression, the performance of flax is a bit lower, thus the flax ampliTex® 0°/90° 300 gsm can replace a 410 gsm glass fiber 0°/90° fabric to have the same stiffness. This fabric is ideal to be combined with the powerRibs fabrics 5019 and 5020, replacing a 600gsm carbon fiber layer with same performances in bending.

Amplitex 5057. Considering that glass fibres have a density of 2600 kg/m3 and a tensile modulus of 70 GPa, the flax ampliTex UD 150 gsm can replace a 250 gsm glass fibre UD fabric to have the same stiffness in tension. In compression, the performance of flax is lower, so that the flax ampliTex UD 150 gsm can replace a 220 gsm glass fibre UD fabric to have the same stiffness.

The future products are based on algae or seaweed. The first is a foam and the second is a carbon fibre made from algae.

Checkerspot, a small company has worked on numerous species of microalgae known as phytoplankton. They grow incredibly fast, so they’re nearly infinitely renewable. More importantly, they’re a material that can be manipulated into many of the same products as petroleum. You can convert algae into food, fuel, and plastics.

Checkerspot ferments microalgae to make tailored triglycerides. In one example, the microbes produce triolein, which is made into polyols for polyurethanes. These polyurethanes can form the cores of lightweight surfboards. Checkerspot is working with one of the worlds largest surfboard manufacturers to produce environmentally friendly surfboards. The foam is a major component of surfboards.

In addition to the surfboard application, the company is working with the Swiss textile chemistry firm Beyond Surface Technologies on a water-repellent coating for outdoor apparel. The market, Dimmler (CEO of Checkerspot) says, is shifting away from fluorine-based chemistry. Checkerspot is also working on a specialty composite material for the skiing market.

The algae based carbon fibre product is being SGL Carbon (already conventionally manufactures carbon fibre products). While we’re still waiting for graphene to grab the world’s attention, it’s safe to say carbon fiber manufacturing is a mature industry, with products ranging from bicycles to Boeing jets. A German company called SGL Carbon (SGL), which specializes in carbon and graphite materials and composites, is working on an algae-based carbon fiber that is identical to the current material. The idea is to turn green algae into a sink for carbon dioxide to produce glycerin, which can be processed into the organic compound acrylonitrile – the building block for carbon fiber. Additional R&D is focused on combining these algae-based carbon fibers with hard rock to produce novel construction materials that are lighter than aluminum and stronger than steel. There are no commercial products as of yet.

 

OldMulti; Correct me if I'm wrong but one of the big pluses of Basalt fabric is not in its tensile strength but in it's compression strength.

 

Redreuban. Yes, basalt has good compressive characteristics but as a recent study by Mafic in collaboration with the Fraunhofer Project Center (London, Ontario, Canada) confirmed a higher tensile modulus, tensile strength and interlaminar shear strength, a 40 percent higher specific strength and a 20 percent higher specific stiffness for basalt fiber/epoxy test panels compared to E-glass/epoxy panels made with the same resin and fabrication process. In short you get good compressive strength and other better structural characteristics than E glass.

Also important in some applications are the differing failure modes of carbon and basalt. While carbon fiber, when damaged, tends to “shatter” catastrophically and sometimes in more than one place, basalt fiber experiences what could be characterized as a gentler failure mode. Also, characteristics that may come into play include added mechanical performance such as stiffness and strength, resistance to impact, chemicals, corrosion and water as well as a difference in failure mode compared to glass, which tends to splinter more than basalt.

Basalt is a high strength and high modulus fiber that is a low cost alternative to S glass and can replace carbon fiber in some applications. It has high temperature resistance with good fatigue and corrosion resistance properties. Basalt has no need for special processing equipment being easy to handle and process. Basalt fabrics are also compatible with many resins EG polyester, vinyl ester and epoxy.

In Australia Basalt fabrics are priced between E glass and S glass. The chart below gives the idea in simple terms of “relative strengths”.

 

For those who want to do a bit of home design of a multihull I found this interesting little PDF update from the Westlawn Institute of Marine Technology. There are many famous naval architects who have got their start in the marine industry from a Westlawn qualification. Westlawn runs several courses but the one of most interest is Yacht & Boat Design enables students to master the principles of design based on the fundamentals of small craft naval architecture and marine engineering. The program consists of 38 lessons divided into four modules (see syllabus below). The subjects are covered in 24 text books, study plans, and other materials that are sent to you at the beginning of each module. The tuition fee (not cheap over $17,000 US for this course) includes the cost of these materials. Westlawn instructors will provide you with the guidance you need to become proficient in each subject so you are prepared for a career as a professional yacht and small-craft designer with EG production boat building companies, independent design firms, and as self-employed yacht designer. The Yacht & Boat Design course covers:

Module 1: Principles of Small Craft Naval Architecture
Introduction to Yacht & Boat Design
Basic Mathematics
Introduction to Hydrostatics
Review
Principles of Resistance
Stability Part 1 and Part 2
Design Practicum
Review
Introduction to Marine Drafting
Drawing of Lines
Module 1 Examination

Module 2: Boat and Yacht Design - Powerboats, Sailboats and Multihulls
Exterior and Interior Design
Design Practicum
High Speed Power Boats Design
Practicum
Sailboat Design Parts 1 and 2
Design Practicum
Multihull Design
Module 2 Examination

Module 3: Boat and Yacht Construction
Wood Boat Construction
Design Practicum
Fiberglass Boat Construction Part 1 and Part 2
Design Practicum
Fiberglass Boat Building - Production Methods
Aluminum Yacht Design & Construction
Introduction to Computer Aided Yacht Design
Module 3 Examination

Module 4: Marine Systems Engineering
Marine Engines
Propulsion Systems
Electrical Systems
Systems & Equipment
Specifications
Professional Practice
Final Examination

The attached PDF is just a minor update of multihull design as an example of what Westlawn does and requires you to understand.